Horizontal wells are being widely used throughout the oil and gas industry to enhance project economics and to develop reservoirs that would otherwise not be commercially viable. Well productivity can be increased with horizontal wells and many fields have been developed for this reason. Horizontal wells have been completed in high productivity reservoirs for the purpose of reducing gas and/or water coning, thereby improving drainage efficiency and ultimate recovery.
Horizontal wells have been drilled and completed in poorly consolidated formations that typically have high permeability and high production potential. These formations, however, are often incapable of producing without some type of control technique for sand and other fine solids. Sand and fine solids produced with oil and gas are major causes of uneconomic production, resulting in excessive expense as well as wear and down time on pumps, tubing, traps and other equipment. The industry has recognized the use of gravel packed completions as a solution to sand control problems.
In normal vertical wells, the cost of gravel packing is usually less than ten percent of the total well cost. Because the benefits of a successful gravel pack include unrestricted productivity, long term performance and selective production capability, the decision to gravel pack is normally made without much difficulty. However, faced with production intervals from 10-30 times the typical completion length in a vertical well, operators must give extra consideration in horizontal wells where the completion cost could equal or exceed the drilling cost.
A conventional gravel packing technique involves locating a perforated liner at a subsurface location in the well and thereafter placing gravel around the perforated liner. A slurry of gravel suspended in a liquid carrier is pumped into the annular space between the formation wall and the liner. As the suspension reaches the bottom of the annulus the gravel is compactly deposited in the annulus on the exterior of the liner and the liquid carrier withdraws through the liner perforations and back up the casing string. In this manner the gravel progressively builds up in the annulus surrounding the liner.
A problem encountered with this technique arises when the well bore deviates from the vertical. When the well is inclined, the gravel oftentimes fails to pack uniformly, resulting in voids within the packed annulus which weaken the pack and permit the production of sand entrained fluids.
Various alternatives to such open hole gravel packs include the use of prepacked liners, i.e., gravel prepacks. A conventional gravel prepack has a layer of uniformly sized gravel contained between concentric screens such that fluid flow must pass through the gravel to enter the well bore. The gravel prepack may contain either loose or consolidated gravel, the latter offering more protection from erosion because the sand grains cannot settle, reorient or move which could allow formation sand to penetrate into the well bore. The screens may be, for example, perforated tubes or wire wrapped screens.
A problem with conventional gravel prepacks is their inability to pass through a severe dog-leg in the well. Many conventional gravel prepacks are too rigid to withstand deviations over 10-12 degrees per 100 feet.